1. Field of the Invention
The present invention relates to image readers capable of automatically reading microfilms, and particularly to a line sensor for automatically reading microfilm using a line sensor with low cost and capable of obtaining high resolution.
2. Description of the Background Art
A micro-scanner is known as this kind of conventional image reader, for example. The micro-scanner includes a film carrier for holding a microfilm permitting it to run, an optical projecting system for radiating light upon the microfilm, a screen on which a micro image of the microfilm is projected, and a line sensor for reading an image projected on the screen. An operator retrieves a desired micro image and projects it on the screen. The operator then confirms the projected image and then scans and reads the micro image with the line sensor.
In micro images frame images) in a microfilm in its elongated direction, positions in its width direction may deviate, or the sizes may not be uniform on the microfilm due to photographic errors and the like.
Accordingly, with conventional micro-scanners, an operator has made positional adjustment of microfilm to locate a projected position of a micro image at a center position on the screen, or has made adjustment of projecting magnification to form a projected image with appropriate size. Then, after performing the adjustment, the micro image is read by using a line sensor.
Recently, however, with development in automation in offices, there is a demand of reading micro images by remote operation. In such a case, in place of positional adjustment of microfilm and adjustment of projecting magnification made by an operator, variations of photograph positions of micro images, variations of image sizes and the like must be automatically corrected. That is, without such correction, a line sensor reads a frame image with its part being out of its reading area, resulting in lack of part of the frame image in a read image.
FIG. 14 is a schematic diagram illustrating positional relationship between a CCD line sensor 140 and a projected image projected on the CCD line sensor 140, and image information read in respective conditions (1) and (2). Normally, as shown in FIG. 14, the length 140a of pixel line of CCD line sensor 140 is set to approximately the same extent as the width of the projected image of frame image I projected on the sensor. Accordingly, if the frame image I is formed at approximately center of the microfilm 141 with an appropriate size and without inclination, the width of frame image I approximately coincides; with the reading area 140a in a main scanning direction (the direction of arrangement of pixels) of CCD line sensor 140. If the frame image I is projected without any positional deviation in the main scanning direction with respect to line sensor 140, line sensor 140 can precisely read the entire image as to the frame image I.
However, for example, due to deviation of image position on microfilm 141, positional deviation in the main scanning direction when projecting an inclined image while rotating the image by skew correction, or positional deviation resulting from adjustment error between a film carrier and line sensor 140, frame image I may not coincide with reading area 140a of CCD line sensor 140 in the width direction (the main scanning direction) as shown in FIG. 14 (1) or (2) when projecting frame image I of microfilm 141. As a result, frame image I is read by CCD line sensor 140 with its part being lacking. A.sub.1 and A.sub.2 indicate lacking portions of read images.
Therefore, in conventional image-scanners, such problems have been solved by using the following methods.
That is to say, (1) as shown in FIG. 15, the magnification of a photographic lens is adjusted to low magnification in order to prevent lack of frame image I projected on CCD line sensor 140 when reading. Only a frame image portion is extracted from image data of read image IR and further the size of image is enlarged on the image data. Otherwise, (2) as shown in FIG. 16, using CCD line sensor 140 having a long reading width 140b in the main scanning direction, lack of projected frame image I in reading is prevented even if a position of frame image I deviates. Then, image data only of a frame image portion is extracted from image data of read image IR.
Correction about position and size of frame images have been made to remove no-image portions by either one of above-mentioned methods.
In such image scanners, shading correction has been made in order to correctly reproduce frame images. This is mainly for correcting illumination unevenness (optical unevenness caused by a light source and the like) of optical projecting system 13 of an image scanner, and unevenness of outputs of respective pixels (unevenness of outputs caused in a process of manufacturing CCD line sensor, for example) of CCD line sensor 11. For the shading correction, an image scanner is provided with a shading RAM, in which data for shading correction is stored. Then, the shading correction has been applied using the data for shading correction to a detected image signal.
In such conventional micro-scanners, however, the following problems have been occurring because of the above-described methods (1) and (2).
That is to say, according to the method (1), since a frame image is extracted from a read image and subjected to an enlarging operation, its resolution decreases. Also, according to the method (2), although the correction can be made without decreasing resolution, there has been a problem of an increased cost for manufacturing the entire scanner since it needs a CCD line sensor with a large size (having a large number of photoelectric conversion elements). Furthermore, there is also a problem that image data processings such as trimming is required.